[PATCH 1/2] md: Add PCI_P2PDMA support for MD RAID volumes

[Christoph Hellwig]

On Mon, Mar 23, 2026 at 04:44:15PM -0700, Chaitanya Kulkarni wrote:
> MD RAID does not propagate BLK_FEAT_PCI_P2PDMA from member devices to
> the RAID device, preventing peer-to-peer DMA through the RAID layer even
> when all underlying devices support it.
> 
> Enable BLK_FEAT_PCI_P2PDMA by default in md_init_stacking_limits() and
> clear it in mddev_stack_rdev_limits() during array init and 
> mddev_stack_new_rdev() during hot-add if any member device lacks support.
> 
> Tested with RAID arrays containing multiple NVMe devices with P2PDMA
> support, confirming that peer-to-peer transfers work correctly through
> the RAID layer.

Which personalities did you test this with?  Parity RAID needs to
copy from the data payload, which is not going to work very well
with P2P mappings.